Populations and Carrying Capacity
What kinds of things can influence the
size of a population of living things in an
ecosystem?
Agriculture and Humans
- For thousands of years,
the human population
on earth remained quite
stable
- But when human societies switched from
hunting and gathering to agriculture,
human population began to grow rapidly.
- This is because societies that harvest and
store food during the winter months or
during periods of drought have an
advantage over those that seek food
every day.
Population Growth Explosion
By the mid-1600s, human
population reached 500
million. After that human
population grew rapidly.
At the present time, about 80 million
people are added to our population every
year. There are more than 7 billion people
on Earth today.
Natality
The population of a community can
change in size in different ways. One of
the ways is natality which means birth
rate. If the birth rate increases, then the
population will increase also.
Immigration
Immigration refers to the number of
individuals of a species moving into a
population. This can also increase the
number of individuals.
Mortality
A population can also decrease. mortality
refers to the death rate. The higher the
mortality rate, the lower the population
becomes.
Emigration
Emigration refers to the number of
individuals of a species moving out of an
existing population. This decreases the
population of the species.
Population Growth
If we take into account the last 4 ideas, we
can say that the population growth of a
species in an area can be written as
Population growth =
(births + immigration) – (deaths + emigration)
Limits on Population Size
Let’s suppose that a
species had everything
it needed to grow and
reproduce. For example,
mice can reproduce very
quickly. It takes only 6 months for a new
born mouse to be ready to reproduce. A
mouse can have a litter of 6 or more pups
and the mouse can reproduce every 6
weeks. If we started with 20 mice, there
could be 5120 mice in only half a year!
Biotic Potential
So what limits the
population size of a
species. One of the
factors is
biotic potential which is
the maximum number of offspring that a
species could produce if resources were
unlimited. Biotic potential includes
1. birth potential, 2. capacity for survival, 3.
procreation, and 4. length of reproductive
life.
4 Biotic Potentials
The following table is a good comparison
between 4 factors that affect biotic potential.
Environmental Factors
In addition to biotic
potential, environmental
factors also limit how
large a population can
become. For example a
fern plant produces more than 50 000
spores in a single year. If all the spores
germinated, fern plants would cover all of
North America within two generations of
the first plant. Why doesn’t this happen?
Factors That Limit Populations
The following chart is a summary of ways
that abiotic factors and biotic factors can
cause a population to increase or
decrease.
Carrying Capacity
The carrying capacity of a community is
the maximum number of individuals of a
species that can be supported by an
ecosystem.
Carrying Capacity Graph
The number of individuals in a species will
fluctuate around the “carrying capacity.”
Carrying Capacity Example
Suppose that there are
very few predators
for mice. They will
reproduce quickly at
first and rise above the
carrying capacity. But then the mice will eat
all the available food. They may become
sickly making them easy prey for hawks,
owls, and foxes. The mouse population will
decline to below the carrying capacity. Over
a long time, the population will fluctuate
around the carrying capacity.
Population Density
When there is a large
number of organisms
in an environment, we
say that the population
is dense. In the photograph at the right,
there is a “dense” population of penguins.
Density-Independent Factors
Density-independent
factors affect the
members of a
population regardless
of the population density. Examples would
be drought, a severe winter, hurricanes,
and fire. These will affect the size of the
population regardless of how large the
population is.
Density-Dependent Factors
Density-dependent
factors affect a
population because
of the density of the
population.
For example, the size of a predator
population (like a wolf) is affected by the size
of the prey population (such as rabbits). As
the prey population is increases, there will
be more predators. But more predators
causes the prey population to go down, and
this in turn produces less predators.
Factors Summary
The following is a summary of factors that
can cause changes in the size of
populations.